This! JJ isn't off the hook just cause his life sucks. He has no right to bully and do all the stuff he did to Dan.
Jae's actions were ones he made of his own free will. He wasn't awful to just Dan, but almost everyone around him, though Dan definitely took the brunt of it. Probably his other sexual partners as well. Before these flashbacks, I was seeing smidgens of Jaekyung realizing that, hey, he's an asshole. So I continue to await the day that he finds his reason to better himself.
Also, I'm aware this comment is probably gonna upset some people
They get upset cause people think just because you gave a background to a person's behavior or gave a reason to their actions it's forgiven. That's not how it works. He's not being forgiven and shouldn't be but, change is a thing. Given his past and hoping in future he realizes that he's doing the same thing he hates in all his life, He'll change. But, I think he is changing and Dan is just the key to that.
While one's past may explain their current behavior, it does not excuse it. I'm seeing a lot of people excusing Jaekyung actions after these past couple of chapters. While his past is indeed awful, I will not view him differently until I see improvement in his present-day self. This won't be an immediate switch, but a step-by-step process of actually trying to do and be better.